How much gift tax will I have to pay if I give my daughter $100,000?

None. At least none right now. You would have to file a gift tax form with the IRS (required if you make a gifts of over $12K in a year to a person other than your spouse). The amount over $12K is applied against your lifetime gift exclusion of $1M. So in your case you that leaves about $912K still left over for gifts during your lifetime before you have to pay any gift tax. It also reduces by $88K the amount of exclusion your estate would have from estate taxes when you die.